1. Field of the Invention
The present invention relates generally to a method for forming a capacitor of a semiconductor device, and more particularly to a method for forming a capacitor of a semiconductor device, which can prevent defect occurrence due to infiltration of an etching solution into a TiN layer applied as material of a cylinder-type storage electrode.
2. Description of the Prior Art
With the rapid increase in demand for semiconductor memory devices, a variety of technologies have been proposed to obtain a high-capacity capacitor. In general, a capacitor has a structure in which a dielectric layer is interposed between a storage electrode and a plate electrode, and its capacity is proportional to an electrode surface area and a dielectric constant of a dielectric layer while being inversely proportional to an inter-electrode distance, that is, a thickness of a dielectric layer. For obtaining a high-capacity capacitor, therefore, it is necessary to use a dielectric layer having a high dielectric constant, to enlarge the electrode surface area and to reduce the inter-electrode distance.
However, since there is a limit to the reduction of the inter-electrode distance, that is, the thickness of a dielectric layer, current researches for ensuring sufficient charging capacity are progressing toward the enlargement of the electrode surface area and the development of a new dielectric layer having a higher dielectric constant.
For example, whereas the existing storage electrodes have employed a concave-type structure in which only inner surfaces of the electrode are used, the latest highlighted storage electrodes employ a cylinder-type structure in which outer surfaces as well as inner surfaces of the electrode are used together and thus its electrode surface area is enlarged. Also, ONO has been used as the existing dielectric layer material, but a single layer or a laminate layer of high dielectric constant material(s) such as Al2O3, Ta2O5, HfO2 and the like is in the spotlight.
Moreover, research for ensuring sufficient charging capacity aims at the development of not only the dielectric layer itself, but also the electrode material used. To be specific, polysilicon has been mainly used as storage electrode material, but vigorous research is currently being pursued to apply metal, such as TiN, as the storage electrode material. This is because TiN is easy to remove a native surface oxide and can sufficiently reduce an effective oxide thickness whereas polysilicon is limited in reducing the effective oxide thickness due to the native surface oxide.
However, when a TiN layer is applied as the storage electrode material to form a cylinder-type structure, the following problem occurs:
In general, in order to form a cylinder-type capacitor, a wet etching process called ‘dip-out’ is needed to remove a mold insulating layer used for obtaining a cylinder structure after a cylinder-type storage electrode has been formed. However, when a TiN layer is applied as the storage electrode material, defect sources such as pin holes or micro cracks which may exist within the TiN layer cause a defect that a lower oxide layer (that is, insulating interlayer) or a lower storage node plug material that is, polysilicon) is lost in the course of the dip-out process.
Here, the defect occurring in the dip-out process can be divided into two types. One type is that the lower oxide layer is lost directly due to an etching solution infiltrating through local defect sources which exist within the TiN layer, and the other type is that composition of BOE which is used as chemicals in a bottom portion inside of a narrow contact hole for the storage electrode changes to increase a concentration of NH4F relative to HF, so that the lower storage node plug material, that is, polysilicon coming in contact with the local defect sources of the TiN layer, is lost first and then the oxide layer around the lower storage node plug material is lost.
FIGS. 1 to 3 are photographs and a corresponding sectional view showing defect occurrence of the former type and FIG. 4 is a photograph showing defect occurrence of the latter type.
Referring to FIGS. 1 to 3, it can be seen that an etching solution infiltrates into a lower oxide layer, that is, an insulating interlayer through defect sources such as pin holes or micro cracks existing within a TiN layer as storage electrode material, which causes a defect that the insulating interlayer is lost.
Referring to FIG. 4, in the course of a dip-out process, concentrations of NH4F and HF constituting a 20:1 BOE solution may deviate from an equilibrium state in a bottom portion inside of a narrow cylinder with the result that a concentration of NH4F becomes higher than that of HF. In such a condition, as defect sources such as pin holes or micro cracks exist within a TiN layer, a reaction according to the following reaction formula may occur in a region coming in contact with a lower storage node plug consisting of polysilicon, so that the storage node plug may be lost and subsequently an insulating interlayer may be removed to produce a circular defect.Si+4OH→Si(OH)4Si(OH)4+4HF+2NH4F→(NH4)2SiF6+4H2O
In FIGS. 1 to 4, reference numeral ‘1’ designates a semiconductor substrate, reference numeral ‘2’ designates the insulating interlayer, reference numeral ‘3’ designates the storage node plug, reference numeral ‘4’ designates an etching stopper nitride layer, reference numeral ‘5’ designates a cylinder-type TiN storage electrode, reference numeral ‘A’ designates the defect source, reference numeral ‘B’ designates a bunker defect and reference numeral ‘C’ designates the circular defect.
In a case where a Ru layer, instead of the TiN layer, is applied as storage electrode material to form a cylinder-type structure, the above-mentioned defect also occurs.